Register | Sign In

Understanding through Discussion

EvC Forum active members: 57 (9173 total)
2 online now:
Newest Member: Neptune7
Post Volume: Total: 917,583 Year: 4,840/9,624 Month: 188/427 Week: 101/85 Day: 6/2 Hour: 0/0

Thread  Details

Email This Thread
Newer Topic | Older Topic
Author Topic:   An ID hypothesis: Front-loaded Evolution
Member (Idle past 1831 days)
Posts: 72
From: United States
Joined: 06-30-2011

Message 29 of 216 (653269)
02-19-2012 4:37 PM
Reply to: Message 1 by Genomicus
02-19-2012 5:43 AM

Some objections...
Genomicus writes:
The front-loading hypothesis proposes that (a) early in earth’s history, the earth (or the solar system) was intentionally seeded with unicellular life forms (i.e., directed panspermia) and (b) these life forms contained the necessary genomic information to shape future evolution, such that the course of evolution was biased in pre-determined trajectories. Thus, evolution would be biased by the genomic information designed into the first genomes on our planet. There is evidence for the notion of panspermia, which I will discuss briefly. A genomic clock based on increases in genome sizes throughout the history of life on earth suggests that life may be roughly 10 billion years old (Sharov, 2006), which would indicate that the first organisms from which all present taxa descended did not originate on earth.
Panspermia, of course, pushes the problem one level deeper. Would it necessarily follow that the propagators themselves (of which you talk of) originated by way of earlier directed panspermiatic projects? Where does the chain end, if it ever ends?
Front-loading does not propose that all aspects of evolution were programmed and determined. There would be nothing stopping the blind watchmaker from taking its own unplanned courses alongside the front-loaded objectives. What might these objectives be? To use front-loading as a working hypothesis, it is assumed that multicellularity was an objective of the front-loading designers, as well as the origin of animals and plants. Further, the front-loading hypothesis proposes that the designers were rational agents; thus, poor, sloppy design in a biological system would count against the thesis that that system was designed into the first genomes.
And what's so special about multicellularity, then? The vast bulk (by mass) of life on Earth has been, and currently remains, unicellular (bacteria in particular). If multicellularity (and therefore plants and animals and fungi) was a legitimate goal, why not have started off with it immediately?
The genetic code is highly optimized for error minimization (Freeland et al., 2000). This optimal genetic code is nearly universal across all taxa. Curiously, there is no phylogenetic tree consisting of less optimal codes present in basal lineages, with more optimal codes being in late-branching taxa. This is interesting because if the genetic code evolved gradually, starting with less optimal codes (there are far more sub-optimal codes than there are optimal codes) which were gradually fine-tuned to produce the universal optimal code, we might expect such a phylogenetic tree to exist. Arguing that the sub-optimal codes once did exist early in life’s history, but vanished once the optimal codes came on the scene (i.e., that they were outcompeted), looks awfully ad hoc.
Unfortunately, constructing phylogenic trees requires the existence of intact genetic material, which by and large is not preserved over billions of years; and thus is necessarily limited to currently-living species.
Significantly, the fact that less optimal genetic codes do exist in nature (see Freeland et al., 2000; note that these sub-optimal codes are secondarily derived — that is, they evolved from the canonical genetic code, not the other way around) is proof-of-concept that the universal optimal code can exist without causing less optimal codes to vanish from the scene. The below image illustrates a hypothetical phylogenetic tree consisting of less optimal genetic codes in deep-branching lineages. It exemplifies what could have been the case, with different codes evolving into more optimal codes.
That the various extant alternative genetic codes are derivative from the "universal" code is quite uncontroversial, given that they seem more likely to have arisen via extreme genetic bottlenecks. Unfortunately, this says little to nothing about the actual origins of the universal code itself, except that it shows that even something as hard-wired as the universal genetic code is subject to a possibility of some change under the right circumstances. It is probably reasonable to infer that all known extant life forms one clade with its last common ancestor having had that particular code, but it does not preclude the past existence of other contemporaneous (and currently extinct) clades which had competing codes.


This message is a reply to:
 Message 1 by Genomicus, posted 02-19-2012 5:43 AM Genomicus has replied

Replies to this message:
 Message 32 by Jon, posted 02-19-2012 4:52 PM DWIII has not replied
 Message 42 by Genomicus, posted 02-19-2012 5:43 PM DWIII has seen this message but not replied

Newer Topic | Older Topic
Jump to:

Copyright 2001-2023 by EvC Forum, All Rights Reserved

™ Version 4.2
Innovative software from Qwixotic © 2024